Spatiotemporal and hydrodynamic influences on microbial and exometabolite dynamics in coral reef and seagrass ecosystems.

Coral reef and seagrass ecosystems provide critical storm protection and economic revenue to tropical coastal communities, and therefore, effective monitoring and restoration strategies are essential. Microorganisms, and the metabolites they produce and consume, are key drivers of coastal ecosystem function. However, microbially mediated metabolite recycling remains poorly understood, limiting its inclusion in conservation and restoration strategies. Here, we examine how seawater exometabolites and microorganisms vary in coastal ecosystems, across spatial and temporal scales and in relation to hydrodynamics. We characterized benthic seawater from two St. John, US Virgin Islands coral reefs (Yawzi and Tektite) and one seagrass meadow at dawn and mid-day over four consecutive days in January 2021. Using quantitative metabolomics and small subunit ribosomal RNA gene amplicon sequencing, we found that exometabolite and microbial community composition differed between sites. By applying hydrodynamic modeling, we determined that the daily changes and system variability were strongly influenced by water source origins. Mid-day offshore water intrusion at Yawzi reef likely drove exometabolite and microbial shifts toward oligotrophic taxa (e.g. SAR11, SAR86), whereas a high percentage of coastal source water in the seagrass site maintained stable exometabolite pools and supported diverse microorganisms. These findings demonstrate that geographically constrained site-level differences and hydrodynamics significantly impact exometabolite and microbial assemblages over short timescales. Integrating exometabolites, microorganisms, and hydrodynamics provides new insights into coastal ecosystem functioning useful for environmental monitoring and restoration strategies.